The present invention relates to a gripping device and a method for gripping sheet material, in particular a sheet-material stack, as well as to an apparatus which has the gripping device.
For transporting a stack of sheet material from a starting position to a target position there are known stationary transport devices which convey the entire stack from a starting position to a target position. Alternatively, it is also known to draw off the individual sheets individually one after the other from the sheet-material stack at the starting position and to convey them to the target position and to stack them up there again, where applicable. Since such apparatuses are elaborate and, with regard to a variable starting and target position, inflexible, in some applications the respective sheet-material stack is transported manually by a person. Instead of a manual transport it is also known, however, to employ a gripper which can grip the sheet-material stack and transport it to the target position. The gripper compresses the sheet-material stack at its upper and lower side, in order to achieve a grip-clamping of the sheet-material stack. Such grippers can be employed in particular for removing a sheet-material stack from a container and depositing the removed sheet-material stack at a desired target position.
For the processing of value documents there are used, for example, containers which are openable on one side or are permanently open, and into which one or several value-document stacks of loose value documents can be inserted. For removing the value documents, the container is usually disposed such that its open side points upward and the value documents stand in the container on their longitudinal edges.
When several value-document stacks are to be kept separated from each other in the container, e.g. to permit that the stacks can be distinguished from each other, the container can be provided with separating elements which divide the container into several partial regions, cf. FIG. 1a. The inner wall of the container as well as the separating elements usually have a meandering surface comprising several projections and recesses at their sides facing the value-document surfaces. To automatically remove the value-document stack from the container by a gripper, a gripper having several gripping fingers can be used, which penetrate into the recesses of the meandering surface from above. Accordingly, the recesses of the container and the gripping fingers of the gripper are adapted to each other with respect to their arrangement and size. The gripper has e.g. two rake-like gripping elements comprising several gripping fingers which can be moved toward each other and away from each other, in order to grip the value-document stack as soon as the gripping fingers have penetrated perpendicularly into the recesses of the container. With the aid of the gripper the desired value-document stack can thus be gripped, removed from the container and be deposited remote from the container or inserted into an apparatus for value-document processing.
A trouble-free removal of a sheet-material stack from the container in this way, however, is only successful with those sheet-material stacks in which the number of sheets is not too large. Because large stacks, due to their larger mass and inconvenient geometry, are susceptible to fall from the grip-clamping of the gripper when these stacks are removed. It is furthermore particularly difficult to reliably remove mixed stacks comprising sheet material of different sizes, because this leads to non-uniform mechanical properties of the sheet-material stack. A further difficulty results from the fact that the force which the gripper must exert on the stack surface in order to hold the stack firmly against gravity and against accelerating forces depends on the friction between the gripper and the sheet located at the stack surface as well as on the friction between the sheets of the stack, and that these friction forces vary depending on the type and state of the sheets. A reliable and trouble-free removal of sheet-material stacks has thus hitherto been possible only for certain types of sheet-material stacks.